A Cationic Pt(II) Dinitrogen Adduct: Synthesis and Characterization of a CCC-aNHC Pincer Pt Complex with Abnormal N-Heterocyclic Carbene Donor Arms

Abstract

In contrast to the reported CCC-NHC pincer ligands that contain normal N-heterocyclic carbenes (NHC), herein we report an imidazole-based abnormal NHC (aNHC) pincer ligand, CCC-aNHC. The CCC-aNHC pincer Pt complex with two aNHC donors was synthesized via the in situ metalation and transmetalation methodology. The 1,3-phenylene(bis-2phenyl-3-butyl imidazolium) diiodide salt was reacted with Zr(NMe 2 ) 4 to generate a CCC-aNHC pincer zirconium complex in situ. It was transmetalated to Pt using [Pt(COD)Cl 2 ]. Electrospray ionization of the Pt pincer complex [( Bu C a-i C a-i C Bu )-PtI] in acetonitrile generated an intense peak at m/z = 696.2375, which was assigned to the dinitrogen adduct [M−I+N 2 ] + of the cationic CCC-aNHC pincer Pt(II) complex [( Bu C a-i C a-i C Bu )Pt− N 2 ] + , representing a rare example of the platinum dinitrogen organometallic complex. The super electron-donating ability of the pincer ligands with abnormal NHC enabled the cationic CCC-aNHC pincer Pt(II) complex to selectively bind N 2 over MeCN in a first-order analysis. A collision-induced dissociation (CID) study was conducted on the N 2 and MeCN adducts, suggesting that more energy was required to dissociate N 2 than MeCN. A computational study suggested that the N 2 adduct was kinetically stable in the gas phase whereas the MeCN adduct was thermodynamically preferred. The computational results reconciled the mass spectral data experiment with an attempt to isolate the N 2 adduct. DFT computation suggested that N 2 dissociation is more challenging due to higher energy transition states, and there is a competitive pathway of N 2 tumbling within the coordination sphere of the Pt. This tumbling path is not available from the MeCN ligand due to ligand structural differences.